ntu_pairs = (
    (1, 2), (2, 21), (3, 21), (4, 3), (5, 21), (6, 5),
    (7, 6), (8, 7), (9, 21), (10, 9), (11, 10), (12, 11),
    (13, 1), (14, 13), (15, 14), (16, 15), (17, 1), (18, 17),
    (19, 18), (20, 19), (22, 23), (21, 21), (23, 8), (24, 25),(25, 12)
)
